Diagnosed with a really bad uti infection, now got a lot of ulcers. Recently was checked for herpes and came back negative, what else could it be?

Probably herpes. What does "recently" mean? Having lots of ulcers suggests newly acquired genital herpes. A negative test even a few days ago may be meaningless. The ulcers themselves need to be swabbed to check for HSV. Do it ASAP, today if possible -- the sooner the better for accurate results. Also insist on a prescription for anti-herpes Rx like valacyclovir; don't wait for test results. Good luck!