The 4 months we've been ttc i've had shorter cycles n very light periods, now this past 5th cycle was longer w heavy bleeding. stress? miscarriage?

Normal variation: It is not unusual to see some variation in cycles, nor is it unexpected to have a heavier period following several light periods. I've seen many conceptions occur when the couple had given up trying, so stress is certainly a factor though it is very hard to measure. Heavier bleeding after a longer cycle could be a miscarriage, but is difficult to prove and has no practical meaning. Keep trying!