False positive?

So I just took a frer test and within 5 minutes I saw a very faint line. The last time my husband and I had intercourse was 21 days ago... I'm not sure when or if I ovulated, we have been taking some time off, so I haven't been logging anything. But my question is, is it possible I'd have a legitimate positive 21 days later? Especially because it is so faint... I test every 4-5 days because my cycles are so irregular, and up to this point all I have had have been negative. I'm planning to check again with fmu, but this has to be a false positive, right? I don't want to get excited because we've had such a rough two years trying.

A little about this cycle... I've had very light spotting for the last 2weeks, but rarely heavy enough to need a pad. I am also on cycle day 79 as of now...