BFP then fainter lines?

I recent had a very faint line, continued testing, the lines got dark (over 3 days) and this morning the line was fainter than the day before. I keep testing because it will be a while before I can see my doctor (he's out of town), so I've been testing with FMU to monitor my hcg levels. I am absolutely terrified that this won't stick and is a chemical after being on Femara and TTC after a year and a half. Has anyone experienced this?