We are sorry to hear of your difficulties conceiving.
Six days into the cycle is very early to ovulate and can be associated with medical difficulties conceiving that are unrelated to mikveh, in particular because the uterine lining is not thick at that stage of the cycle and the egg is immature.
Please see a physician as soon as possible in order to confirm your timing, and to discuss possible causes and treatment, including the medical possibilities for pushing off ovulation.
In the meanwhile, you can learn more about ovulating before immersion in our article here. In some cases, performing an earlier hefsek taharah is possible, but we need more information to make that determination.
Once you have a clearer picture of the medical situation, please be in touch with us, so that we can offer you appropriate halachic guidance. You may find it helpful to consult a Yoetzet Halacha Fertility Counselor. (This is a free service, details here.)
This response was updated on 20 November, 2020.