I have a kitten born 3/24 this year and she has definitely gone into her first heat. She is starting to accept the advances of my stud, and I now have to separate them. I was told by my mentor that she could be bred during the second heat, but I was wondering if that is still too early? When is it normally best to breed a new queen?

She would be far too young to be bred from on her second heat if she calls again within a few weeks/months.

I would wait until she is at least 9 months old, and then make absolutely sure that she is a very healthy weight and size before you consider putting her to stud.

As your girl is an early caller, keep a close eye on her, and any signs of lethargy, loss of appetite etc... then take her to your vet for a check-up, just in case she has an infection and may need a course of antibiotics.

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Best Wishes,
Maddy

I agree, girls can call early and by her third heat she will still be very, very young. Third heat is only in reference to grown up girls not kittens.

Pyometra is a horrible disease causing infertility and worse case scenario, death, so be very vigilant and as Madeline says take her directly to the vet at any indication she may be unwell.
A vaginal discharge especially if blood stained, pusy or smelly should also alert you to the possibility of pyo, but pyo can occur without any discharge.

If she has been in contact with your stud then make sure she is continuing to call regularly and is not already pregnant.