Can Siamese cats have heat while already pregnant?

I have a blue point male and a tortie point female. She had two heats, then a third, so we thought this third was the time she got pregnant. If so she would be due the first week of November. However, she just had two kittens five days ago, the first week of October. At first we thought they were early and would die. But they are fine, however this means she would have gotten pregnant during that second heat. Has anyone ever heard of a cat going into heat while she was already pregnant? A month ago she would have been pregnant already from the second heat but she was in heat for a week and her and the boy mated numerous times. What is going on?

Siamese cats are no different than any other cat, and I"ve never heard of a cat being in heat while pregnant.

With cats, ovulation is induced only by the act of mating and that ends the heat cycle.

However, cats who are not bred can be in heat more or less constantly, so even if your queen was not noticeably in heat, she may very well have been and that was when the mating occured. IMO, this is what happened.

I just fostered a Siamese cat who was having a "silent heat". I never knew she was in heat at all, and she certainly would have gotten pregnant if she had been exposed to a male.