Hello! I donated my kidney 3.5 months ago and I definitely noticed a change in my period for the first 2 cycles (unfortunately I got my period the day after donating -- but fortunately, I couldn't feel any cramps because of the medication!). For me, it was heavier, I had painful cramps, and it lasted longer than usual.
Were you on birth control before donating? I was - both for contraception and also to regulate my period (I get painful cramps). So, when they made me stop taking them before the surgery, I'd say those first two periods were very much like the ones I experienced in high school (my worst ones). But cycle 3 was not as bad, and I'm still not back on the pill.
Hope this helps.