Just started ttc, irregular & cancer - any suggestions?

Jess
We are just starting to try, not actively, just if it happens it happens, but I was wondering about some things. We always have had unprotected sex. Part of me wonders if I am infertile because it's been 3 years. You think it would have happened by now, I had cancer about 6 years ago ( thyroid) , which they removed it all and have been cancer free since then. I also had to take two radioactive iodine pills to make sure they got it all and to kill anything else that remained.
 My periods can be regular and sometimes I will miss my period by two weeks getting me all hyped up thinking I am pregnant then the next day I get my period. Never fails! 
He would always pull out but now we just started not to, I haven't charted anything but I think i ovulated the other day when we were having sex. He came inside me as well, since then I have been having mild cramps, backache (lower back), I wake up and two hours later I have to take a nap, some mornings I wake up feeling like I'm going to throw up. I take my thyroid medication almost everyday (I do forget sometimes)
 I do want to mention I have a child from a previous relationship (before I had cancer) and she is 8. Should I get tests done to see if I'm fertile? Should we take other measures to be sure it happens?