Jen is correct, but your perceptions are correct, too.
Spasticity is often accompanied by weakness, but not always. The spasticity - which exerts a pull against its opposing muscle groups will make all movement of those groups harder and will cause the muscles to fatigue way too early. When we fatigue too early we are likely to interpret it as weakness.
Jen's advice about stretching is right on!
Quix
Personally when I have spasticity, it causes the muscle to be less strong. Either because it's fatigued from being so tight, or from weakness, I'm not sure. So maybe the answer is both.
You know what really helps me is stretching. It's really difficult to stretch the hip flexors (I have the same problem) but I've found that laying on my stomach will allow them to relax, and then I can bend them to stretch the problem areas.
This link from NMSS has some easy stretches and exercises that will help.
http://www.nationalmssociety.org/download.aspx?id=331
Get your PT to show you some stretches for the hip flexors, and when you know, come back and tell us!