HiCustomer how old is your girl?
How long has this been going on ?
Has she been on a predominantly seed diet ?
Any other birds in the home?
Any chance of injury ?
Is she fluffing her feathers/looking bigger?
Drinking normally? How do her droppings look?
Popcorn ( a terrific name!) has only been with you for 3 months?
This head twitching, is it jerking her head back and seeming to rub the top of it with her wing - or is the wing not involved and she's just jerking the head to one side or the other?
When is the last time she ate and drank normally?
One of the possibilities behind the symptoms you're describing would be a growth internally (tumors are very common in budgies on a mostly seed diet).
When a tumor presses on or invades certain areas of the brain, spine or organs it can cause the twitching you're seeing. Infections (bacterial usually) may also contribute to this problem, along with many other issues.
You need to get her to a vet who will clip a toe nail for a blood sample and check her chemistries, including calcium and magnesium for parathyroid disease (one of those other things that might be behind this). A crop/mouth swab is also necessary - this might be nothing more than a minor fungal/yeast infection (you'll be given appropriate meds, be sure to administer them exactly as prescribed and for the full length of time, no matter what).
When you notice signs of illness or weakness, there are no home treatments or cures and the stuff you'll find pushed in pet stores and online are huge wastes of your money and precious time. No matter what - do not delay having your bird seen. We must never haphazardly treat our birds with medications or herbals without knowing what's really wrong. In doing so, we could make things worse. Much worse.
What you can do while securing a vet to see your companion is supportive care.
Most birds will need a heat source to maintain body heat while you're getting their medical treatment lined up or while you're on your way to see the vet with them.
Putting a heat source into the cage may be necessary since a sick or distressed bird may lose body heat. I prefer a non electric source and use rice socks.
Use a thick, clean sock and fill it ¾ with plain, raw white rice. Knot the end and microwave it for about 1 ½ minutes. Shake it afterwards to distribute the heat and be sure it's not too hot.
You can layer a thick towel on one side of the cage, secure with clothespins out of the reach of the bird's beak - then clip a heating pad over the towel and set on low. Check often to be sure it's not overheating and that the bird isn't gnawing through. A side attachment like this will allow the bird to move closer or away as needed.
If the bird is bottom-bound, install a heating pad under just a portion of the base, again, set on low.
The rice sock option is far safer and more efficient though.
Cover the cage with a thick blanket, leaving all or part of the front door uncovered for better air circulation, to help keep the bird calm by being able to see out and of course so you can check on it regularly just by peeking in. The covering also helps hold some of the heat inside.
I'm very uncomfortable giving the heating pad suggestion because of the potential for the bird to bite through the bars and possibly catch a part of the heating pad. This is just one reason I prefer to avoid electric sources of heat. Another reason is that heating pads can suddenly become faulty and overheat. If ever using an electric source for heating anything in anyway, please be vigilant and constantly double checking carefully.
If your bird will drink and eat on their own, excellent. If not, have an eyedropper ready to administer a few drops of plain water, or better yet, children's Pedialyte ever 20-30 minutes. Put the dropper gently inside the beak and let the drops fall into the bottom beak under the tongue rather than trying to get into the back of the throat. We don't want to chance the bird inhaling the fluid and developing pneumonia.
These are just temporary supportive measures to keep your bird alive while you get professional, hands on care. These measures will not cure your bird.
Don't mistake what looks like recovery to be a real recovery. It is very common for a bird to regain enough strength to start hiding their illness again, but what's happening is that it's progressing and by the time you see symptoms again, it will be much worse or too late.
I'd rather take my bird in for an evaluation and find out everything is fine and there's no known explanation for the period of acting ‘off' (which does happen by the way), than to lose my companion.
Find an avian vet near you http://aav.org/vet-lookup
Many of your local vets might also be qualified to see your bird. Many vets are quite good with smaller birds like this, so ask - and go.
Please let me know how you make out ok?
You can re open this question any time to follow up - and as often as you need to.
Do not press accept again. I'm very happy to help in any way that I can and will be praying for you all!