Blisters/really sore nipples with breastpump??

I've been breastfeeding for nearly 5 months now and still have issues with my Medela pump. I don't know if I just have really sensitive nipples or if it's an issue with the pump. I changed to a smaller size early on which helped, but I can't go more than 10 minutes without blood blisters popping up or clear damage. Ugh... Any advice? I'm willing to shell out $ for a new pump if that's what it takes.