help?!

Hi ladies! 
I got my period 15th of last month and my period is usually a few days late the following month. I woke up this morning (19th March where I am) with dark brown discharge on my undies, and I took it as a sign that it's the first day of my period so I had a pad on the whole day, but all that came out was just a tiny bit of brown discharge?! I also haven't really experienced much pms symptoms this month compared to my other cycles. I usually get really sore breasts and I'd get pretty frequent cramps accompanied by a really sore back. But this time it's just cramps here and there and sore back. Is this brown discharge occurrence normal??? I'm freaking out a little because I had protected sex 10 days ago (9th March) and I've read stuff about implantation bleeding happening around 5-10 days after conception...