I'm really confused (sorry kind of long)

So I'm on birth control (shot) and after I first got on it I missed a shot. But for two months I had a period twice a month. Then I got the shot again in February and haven't had a period since.

Now today I'm cramping slightly and had a little brown discharge but only once. Also I was spotting too but it only happened once and only a little. This came out of nowhere after a hike I took this morning. I've been hiking a month ago and this didn't happen. I do have unprotected sex sometimes.

Anyone know what this could be??