It's always best to bring your cat to the Vet to get checked out... but I've had this happen to my cat and took my cat to the Vet and all they did was use a sterile needle to poke a tiny hole in the abscess and drain the clear fluid and blood from the bump. The Vet told me I could put a warm water washcloth on the area twice a day to keep it soft. He didn't prescribe any antibiotic and said it should clear up on it's own. If it doesn't get better I can always bring the cat back and they could take a scalpel to open up the abscess more.
That being said, the Vet was wrong yet again and he should have taken a biopsy of the "abscess", because I had to go back and see a different Vet who took a biopsy of the bumps. Turns out that it was a Cyst (not an abscess). The cysts were benign, and I was told that the cat should be fine for now. But after a number of months the cyst on his head and 2 on his side continued growing larger, so I decided before they grow too much larger, my cat should have surgery to have them removed. Benign cysts are considered optional for surgery, to have them removed since they typically don't bother a cat, but the cyst on his head continued to grow larger and break out. I wasn't sure if it was infected, and thought it was best to have my cat get surgery to have them removed which cost about $700.