Toddler and potty

Hi all, I have not been on here in a long time, but need help.  I have an almost 3 year old (she'll be 3 next week).  She is ok peeing on the potty, she fights us from time to time though.  My question is about her pooping.  She refuses to go on the potty.  We've been potty training a while, we've had a potty for her since before her 2nd birthday.  I had my 2nd child in May 2014, right after she turned 2.  So we were not forcing the potty training. But after about 6 months, we went hard core on the potty training.  She was difficult, but slowly got better.  She peed and had pooped on the potty.  She'd get a lollipop when she pooped and sometimes M&M's when she peed.  Daycare gave her stickers.  It took a long time before she pooped at the potty at daycare, I think she did it for the first time within the last month.  In the last month or two is when she has stopped pooping on the potty at home.  She holds it or waits until bedtime and then poops in her pull-up.  My husband and I are at our wits end.  We can't get her to poop on the potty and since her pickiness with eating is so tough, that doesn't help either.  We think she gets so constipated with the picky eating and when it builds up and really needs to come out, she just cries and cries.  It can take a good 2 hours for her to actually poop.  Does anyone have any suggestions on what to do or how to help her?  I will be talking to her doctor soon, but any advice would be greatly appreciated.  Thank you!!