Like Tyler said, go and see a dermatologist ASAP. I don't want to worry you, it's probably nothing serious since you're saying the moles are small in size. But melanoma / skin cancer could happen to anyone: young, old, male, female, healthy lifestyle or not. Pale skin is an important factor for susceptibility, though.
So just go and see a dermatologist (or your GP, but chances are he'll just forward you to the dermatologist, too). He/she will check your moles and pigmentation marks, and ask you to come back on a yearly basis. Melanoma is relatively easy to treat, but treatment of big moles could leave some serious scar tissue, so it's better to get them checked when they are small.
And just using sunblock won't completely prevent pigmentation of the skin. It's always better to wear a cap or hat when there's no hair on your head to protect your scalp.