I just found out my cats in heat don't know for how long but she just don't want to eat her cat food . Why?

First of all, when she is finished with this heat cycle, please get her spayed. Sh will be healthier and live longer if you do.

She will eat when she gets hungry. Put out a little bit of food several times during the day to see if she's in the mood to eat. Or, have dry food -- and of course always have fresh water -- available all the time so she can snack when she's hungry.