If you were on progesterone injections and they stop, you should have a period. It might not look like the usual period because it will have been influenced by your hormone treatment in the run-up to IVF. It can take four days to come or even up to two weeks, depending on how influential your regular cycle is. Call your IVF doctor for advice if things seem too unusual. But it is just a period, not cancer.
This can happen for some different reasons. And a hormone imbalance is absolutely one of them. The progesterone injection is a big factor here and I'd say this is normal after that. This does often happen. Hormones are this tricky thing and brown spotting (old blood) is a common thing to have happen. Here's a whole article. https://www.healthline.com/health/brown-vaginal-discharge How long have you been trying to get pregnant?