If you feel 'heat' from the area, you might want to try applying ice or cold packs. If you feel it's muscular (although I understand how difficult it is to know for sure), heat may help relax it.
I don't care for electric sources around animals, but this is up to you too. I use 'rice socks' on our animals (I own an animal rescue org) when they need it.
Use any thick sock, cotton preferred, fill 3/4 with raw white rice and knot the end. Microwave it for 1 to 1 1/2 minutes and then shake it out to distribute the heat and be sure it's not too hot.
This can last for a couple of hours or more if you tuck it under a blanket. You can use two socks just for comfort - cats love the heat.
If you really want to try an aspirin, 1/4 of a "low dose" aspirin may be acceptable; or up to 1/2 of a baby aspirin, but no more than once every 24 hours and never more than two days in a row without veterinary monitoring.
Remember, aspirin is a blood thinner and if there's injury internally, it could make things much worse.
Watch for drooling, vomiting, lethargy, staggering, rapid heart rate, hyperactivity or anything else you know isn't 'right' for the first few hours.
This is a very lucky cat.