A couple days ago I noticed my betta started growing this bumb at the end of its body near the tail, wtf is it? Looks like a big blue pimple! Sorry 4 the crappy pic, best one i could get. The red is a circle i put
Any fish can get a tumor, but it may be that this is lymphocystis, a viral disease that normally won't prove fatal to the fish, though it is contagious.
I'd isolate the fish to avoid spreading to any others, and watch it for a while to see if it grows. Keep water quality pristine to help his immune system stay healthy and strong.
Here are some pics of lymphocystis. It's too soon to tell, and the picture is not very sharp, so I'm not sure that this is what your Betta has, but it could be.
Could you please post a close up pic of the affected area? I'd focus on that area. If it doesn't help to take the photo in the main tank, get a 2.5g tank and place him there so you can get pics properly. It could be tumor or lymphocystis. It's hard to say which exactly does your betta have unless you get closer pics.