Secondary infertility.

I don't understand what is going on. I have a 2 year old, I got pregnant without assistance although it took almost 2 years to get pregnant. Now we have been trying for another 1.5 for baby #2 and cannot seem to get pregnant. We have started to see a reproduction specialist. We did all of our preliminary testing and everything has come back normal, but yet still cannot get pregnant. I was hoping this month would be our month, but I got af today. With the show of af we continue our fertility treatments towards Clomid which I will start in a few days. What is the chances of getting pregnant on Clomid. I am afraid of the side effect, but so want my little girl to have a sibling. I just get heart broken each time I see a pregnancy announcement and when af shows up.