Went three months without a period. then my period started and it was heavy and lasted 6 days with usual pms symptoms. could i be pregnant?

Could be: Do a urine pregnancy test- easy to get over the counter.

When?: There are many things that can delay your period but if it has started and stopped it is unlikely that you are pregnant now. It is possible that you had a miscarriage but doesn't sound likely from what you have said.

No.: Anovulatory cycles followed by an ovulatory cycle , thickened endometrium because of lack of progesteron past 3 months and here we are with a heavy and prolonged period because of thickened endometrium. Nothing here suggest to me that you are pregnant.