Possibility of pregnancy & medicines.

How possible is it to get pregnant while on the pill? I have a few medical conditions; one of them being POTS (postural orthostatic tachycardia syndrome), which among many other things, makes my memory iffy. I take a large combination of medicines and sometimes forget to take some here or there. I'm just very concerned because my cycle is very slight, and most of my medicines are not cleared for pregnancy. My last and only pregnancy ended with PROM at 16 weeks; so I'd want to get off some meds asap.