First, I Would Like To Ask How Long Does Morning

First, I would like to ask how long does morning after pill side effects last? Secondly, I had unprotected sex January 3 and took the plan b January 6. Here it is 8 days later my back is hurting,spotting, breast tender,sleepy etc. I took to pregnancy test both negative. It's only been a week since I took the plan b. Should I wait to take another one? I feel like i maybe pregnant.