Hi Lesilie,
I'd say your quads are too tight or your leg isn't in correct alignment, have you tried to do mini lunges or back lunges?
Let me explain how to do back lunges, and please hold onto the back of the chair the first two times you run threw this, after that, it's your choice to use a chair or not. Okay stand and get ready to go into a normal lunge position. Front leg, the knee should be at 90 degrees, knee should be in a prefect line right over the ankle. Back leg should be as far out as you can go, usually about 3 feet unless your tall, and if your taller you need to slide that back foot back a bit more until you front knee is at a contant 90 degree angle. Your hips should be square and facing forward they should also be a bit more over the back leg instead of the front leg so you should not be leaning either forward or back, your upper body should be straight. Now bend the back leg and go down as far as you can but do not let the knee touch the floor. Then come back up, Your front knee should never move. You can use your glute muscles to sqeeze and push yourself back to starting position.
The other ones, are called half lunges, these are the ones I use for people who have super tight quads.
Normal lunge position, keep the back leg bent just a little, and just go down an inch or two. If these still do not work for you, don't do them as having that back leg stright is bad form and you could possiblity lock that joint one day. That's not a fun experience. But do start doing a ton of hamgstring and quad stretches, as one will help the other.
How well can you stretch your quad? Can you stand in front of a chair and bring up your leg and grab your foot or ankle and stretch the front of the leg? Or do the same thing on the floor which ever is easier. Cathe does some good stretches for the quads and hamgstrings in her new StretchMax, if you are really tight start stretching those areas spedifically at least 3 times a week, when ever you get done working out stretch and hold those stretches for a good 5 seconds or better. The longer the more relaxed the muscle will get and will release all the tension. And keep your breathing even and slow, as that signals the brain to tell the muscle to relax.
Hope that helps,
Kit