I have also got sharp pains in my lower back
If this just happened, you could assume your period is just a little late. If it's been a few days, and if you have been having unprotected sex, take a pregnancy test.
I would also guess your period is coming slowly. Some women do spot before they get their full on period. The pink color is fresh blood mixing with your discharge. Not enough to just be red bleeding so it tinges things pink. Old blood does the same and shows up as brown or tan. So, this is fresh and is likely your period. Let us know what happens with an update. :>)