Why is my Sheltie losing so much fur?

I'm worried something is wrong. She's almost three years old and appears very healthy. She did have two litters in the past year and should be going into heat any day now (she won't be bred this time and the first time was an accident, no lectures please). Her most recent litter was born in October and she only had three puppies (her first was four). She doesn't have much fur to begin with compared to most Shelties. She is losing her fur like crazy and I'm finding clumps all over! You can even pull out tufts of fur near her hind end. I do agility with her and we practice all the time but I couldn't do agility this past month due to an illness and I'm wondering if this is stress related. She is on Science Diet and eats well. She's at a good weight and gets lots of exercise. What could be causing this? Is it hormonal or could it be something more serious? THANKS!