Plan B side effects or pregnancy? I took plan B, I got my period but it was for 6 days

I took plan B about in November, I got my period but it was for 6 days which is unusual for me.

I got my period in December but it was light but lasted 4 days, which was the usual for me?

Now In January, I have the same problem. I haven't gained any weight at all since then. I've taken countless pregnancy tests, but all say negative. I do stress a lot and it wasn't only that situation. I did hear stress can cause belly fat plus i've stopped working out.

I recently lost 40 pounds all together, took me about 4-5 months to do so.

Is the stress the reason why my period is so light now? could I really be pregnant?? or is this just plan b side effects?