My puppy is frequenfly going potty. Both poop and pee about every 10-15 minutes. What could be wrong?

She's a 3 1/2 month old beagle/boxer. I brought her home from a pet store almost 2 weeks ago. I immediately noticed she had diarrhea and took her to the vet within the next couple of days. She had worms and they gave her medicine. The worms went away and for about two or three days her poop became 'normal'. Now for the past 3 or 4 days, her diarrhea has worsened. The consistency is a lot more watery. When she poops she struggles so hard to push only a little bit out. Ive noticed blood in her diarrhea as well. Sometimes its a lot and other times its only a little. And a few times I've noticed that she would poop out straight blood. She whines for me to take her out to potty about every 10-15 minutes. And when she goes it isn't much. She isn't eating as much as she should be but she's drinking plenty of water. I'm taking her to the vet on Thursday. For now I just want an idea of what could be wrong. Please help! Thanks!