Blood in the fecal matter can have a lot of causes. The most common and most likely is a parasite problem. I suggest that you go ahead and see the vet as soon as you can for a fecal exam to rule this out, even though you have done it once. Whip worms are often missed on a fecal exam.
Aside from that, intestinal obstructions and other problems can cause blood in the stool as well.
You can try switching him over to a bland diet of only boiled chicken and white rice until it improves. Then you can start to add his regular food back in.
If he's lethargic, can't keep anything down (even the bland diet) or seems to have neurological symptoms, you need to see your vet soon.
If you switch his food around a lot (treats, table scraps, etc) this might be your cause.
Try the bland diet today and see if it resolves any. If not, or the symptoms worsen, you will need to make an emergency visit to your vet this afternoon.
Does the lump wiggle under your finger like a marble?
Let me know if you still have questions.