light brown bleeding for 13 days and counting, can this be i

My husband and I just started trying to get pregnant this month, and about 3 days after ovulation I started having some really light spotting. Now I've been having spotting/light bleeding for 13 days already... mostly brown but occasionally red... never enough to need more than 1 panty liner in a day (during a "normal" period I usually have really heavy bleeding).
I did a home pregnancy test 12 days after ovulation and it was negative. I'm now 16 days past ovulation and still no real period, just this crazy light, mostly brownish bleeding that doesn't seem to be wanting to stop. I've been reading online and although most people say implantation bleeding doesn't last that long, one woman said hers lasted 3 1/2 weeks, went away for a while then came back for another 3 1/2 weeks.
Has anyone else experienced something like this? Do you think I could be pregnant and maybe this is implantation bleeding, or possibly be having a really early miscarriage, or just a crazy cycle for some reason? Any ideas?
I plan to do another home pregnancy test soon... I'm just waiting a few days since the one I took last week was negative. Thanks in advance!!!