I am about 10 weeks post op and have had the same problem. I had a bilateral and the left quad at the very top, where it attached to the bone was very painful when doing leg lifts. The right was not as bad. I continued the exercises, but did not push myself quite so hard, and took an occasional day or two off. Dr. Gross' protocol calls for me to do them twice per day.
At first it was very discouraging, but in the last 7 days or so I have had a dramatic improvement. I agree with Dr. Gross, walking is one of the best methods of rehab. Although it is more difficult, walking up hills seems to provide quick results. Labor Day, I walked 9 holes of golf and carried my bag. It was a challenge, and I thought I would suffer afterwards, but to my surprise I felt great after only a short rest.
Bottom line, what you experienceing is normal. Keep exercising, but don't over do it.
Mike
Bilateral
Dr. Gross
L 6/23/08 R 6/25/08