if the itching is on your chest too and not the nipples, my guess would be that it is just from the overstretched skin from when your boobs got engorged. And the pimple on your chest... ah those lovely hormonal changes.
Where is the "pimple"? If it's around your nipple, on the areola it could be where milk is coming out, I had a very painful spot on my areola that looked like a pimple, and when I tried to squeeze it a little it shot out milk...I googled it and found it as well...I don't know about being itchy though...