SERIOUSLY HELP ME!!! My MacBook Pro's acting weird!

Ok, I have a SERIOUS problem. I just got 8GB of RAM for my MacBook pro, I checked 4 times to make sure I had the right one, DDR3 1333mHz PC3-10600. I installed it, then turned my computer on. It worked until the login screen. I clicked my name, then a popup came and said "please restart your computer by holding the power button until the screen comes off, and then power it back on." this happened several times, until i decided to go back to my 4GB. I booted it up, then the loading screen with the white BG and grey apple and loading icon came up, and it stayed that way. I then turned it off, then went back to the 8GB again, then the same thing that happened with the 4GB, happened with the 8GB: white BG, grey apple logo, and loading icon. I don't know what to do now as it is still loading as we speak, and I even went to the movies, and came back and it was still loading. Please help me, I don't know what to do and I am scared that I might have messed something up PLEASE HELP ME!!!!!