I've had 3 miscarriages and only ever conciebef while on the pill. I'm 31 and haven't been able to have a baby yet. What could this mean?

Many things possible. There are many causes of recurrent miscarriages, such as genetic disorders (like a chromosomal abnormality), autoimmune causes (like anti-phospholipid syndrome), anatomic causes (such as a malformed uterus), and environmental causes (like smoking). Anyone who has had 3 miscarriages should have a full evaluation by an ob/gyn. Best of luck to you.