I have a 5 month baby I breast feed her 2-3 times rest I give her formula milk , am 7 days late on my periods the HCG strip test came out negative why am I not getting my periods? Am I pregnant?

It is possible to get pregnant while breast feeding an infant. Has your period been regular since giving birth or not? How long have you not been getting any periods?? Also it is normal not to get any periods while breast feeding.
What you need to do is check with your doctor and find out what is going on, since your concerned. Best wishes.