Nipples turn white?

When I first started pumping (manual 2wks ago) everything was quite ok. Now whenever I pump, my nipples become white and cold like all blood just got taken... is this normal? Is there anything I could be doing wrong? I I've pumped 15min per side 2-3 times a day. When I first started off they chipped and was red and swolen. So for it to now be the opposite has me worried.