Nexplanon period

So I got nexplanon birth control at my 6 week PP appointment and I was bleeding/spotting off and on for about a month and a half after having it placed. Then finally all spotting went away. Until now. Welp. It's been 2 or 3 months since I stopped spotting and I haven't had a period at all while on nexplanon. But yesterday and today (bleeding started yesterday) My stomach feels really weird and I am bleeding like a period, but it's bright red and stringy. I'm not sure if this is my period returning (I'm 6 months PP) or something else. It just seems weird that I randomly started bleeding and my stomach is hurting (not like cramps, but like sore and slightly painful).