What Causes Vaginal Spotting And Abdominal Cramps After Blood Donation?

I donated plasma today for the first time at 9am and it now 8pm and I m spotting and experiencing cramps like I m about to start my period. My menes is suppose to start around the 14th. Can this be due to donating blood? And why don t the plasma center tell women about this when their giving all sorts of info and everything else regarding donating?