Yes - the queening heat is what leads to the lovely back-to-back litters that create the population explosion. If you can catch her now, many of the spay-neuter clinics will actually perform an abortive spay (preventing this litter as well as future ones). Or, at the very least, confining her to prevent breeding on her queening heat as you are unlikely to ever find her in a non-pregnant state if not. She is not really feral, just stray, so you won't be likely to qualify for the free catch/release programs many of them offer for ferals, but they are much cheaper than regular clinics - you may also be able to get a voucher for reduced fees on spay/neuter through your local humane society. I'd be happy to help you find information for your area if you would like.