Krys I will put my two cents in here but I could be totally wrong, that happens from time to time

If I remember right spaying a cat in heat is harder on them than waiting until they are not. If so adding her low weight, stress and the condition she came in to me waiting a little while may be best. Not long, just long enough for her to get healthier and out of heat. This is if you have no whole males in the house. JMO

luv reading how much Annabella loves u for saving her and how happy she is now! I'd take her to the Vet and if he or she thinks all things considered it's a good time to spay her then I'd have it done.