No period, negative hpt and now implantation bleeding.

I'm mostly regular on my period. I have it every month give or take a few days. I haven't missed a period since I had my daughter in 2010. I was supposed to start March 28 or so, but I didnt think I was going to start cause I didnt have any of my usual cramps or tenderness. I didnt have my period so I took two tests, with 5 days of missed period. Both negative. April 6 I had implantation bleeding (going off all the signs of implantation bleeding I have read about). I had mild cramps, when I used the bathroom and wiped there was a pinkish tint on the toilet paper. That was it, no more bleeding after that. On top of all this I've had very tender nipples, not my breasts, just nipples; I've noticed that foods and drinks I used to love, no longer taste the same; I am very constipated and pee like a racehorse. So I guess my question is, when should I take another pregnancy test? Could I be pregnant even with the negative tests i took prior to implantation?