Kellie,
I too have experienced this with my pump. I called medela and they told me what may be causing the problem. They said to to take off the peice that you connect the tubing to. Put the pump on minimum and the lowest setting and let the white diaphram part go all the way in then pop the plastic peice back on.
When my pump started doing this I thought that it was dying. Since the last time that I tried this trick I have not had any problems with my pump.
If you have any questions or if I did not explain it good enough call medela at 1-800-Tell-You.
Hope this helps,
Bella