Break-through bleeding in the first 1 - 3 months is not unusual. Nor is a change in cycle length in people who aren't on the pill. It can be shorter, longer, heavier, lighter, skip one, get one early........

You could 1) go get another preg. test & know for sure. Or 2) wait for another 4 weeks & see what develops. Go with your comfort level & either find out or not. I guess it depends on your situation, marital status, and lots of etc.