Bunnies don’t get into heat like dogs do, but they can spontaneusly ovulate and go into false pregnancies, which is very common. How old is your bunny? Uterine cancer is very uncommon in does under 2 years, but before there’s flat out cancer, there are often (perhaps always, but a lot if it is probably asymptomatic and goes undetected) other changes to the uterine tissue such as endometriosis. The cure in any circumstance is to have the bunny spayed asap. If there was cancer and it hasn’t spread, that’s that with the uterine cancer of course. If the cancer has spread, there’s not a lot to be done currently. Uterine cancer tends to spread to the lungs.
I too hope your bunny is ok. Having her spayed was great, it sounds like she had sth wrong with her reproductive organs and those organs are obviously history now, since a full spay means that both the ovaries and uterus are removed.
Is she eating, pooping, normally active?