I am trying to increase the range of motion in my legs. I do many different static and dynamic stretches, 5 times a week.
The next day I often feel mild soreness similar to DOMS. Is this a sign that I am over doing the stretching?
I am getting improvement in my range of motion, but I don't know if this is from days where I stretched heavy or light.
Maybe. I am not sure what you mean by "dynamic stretching" but if you mean bouncing while stretching in any form, then don't do it. You muscles tendons and ligaments have a range of stretch which you are trying to improve. That range has a limit. Beyond that limit, they don't stretch, they tear. The goal is to go to or near that limit for just 30 seconds or so, then rest and let your body adapt during which time it will increase its flexibility.
You know where that limit is because you can feel that tension increase as you approach it. When you can't stretch any further, you're at it. Going beyond this point by bouncing is just a form of throwing your body in a direction which forces the attached muscles tendons and ligaments to extend past that point. Now they're tearing. That's possibly what you're feeling and why.
The response to stretching - increased flexibility - is invoked by staying merely near the limit of your current stretching range for 30-45 seconds. That's it. You do not have to exceed it, you gain no benefit from exceeding it, and you risk micro-injury and the cumulative effects of such injury by exceeding it.
You have to be consistent and work hard for many things in fitness. Flexibility isn't one of those things. To get flexible, you just have to be consistent. Every day, for 30-60 seconds max, stretch until you start to feel a slight clearly non-painful tension that is nevertheless more than you feel otherwise, stay there for 45 seconds or so and that's it, you're done. Changes will happen with all possible speed following that simple and easy regime.
HTH
What Kirby225 said, muscles are like elastic bands. They can only expand so much until the tendons begin to tear. I would look into fixing any posture issues like pelvic tilt first before stretching. Say you have tight hip flexors, the rear or the pelvis will tilt up, elongating the hamstrings close to their tension limit. This can go unnoticed, so if you try to stretch the hamstring further, you might think you are doing a normal stretch, but you would be pulling the tendons since the muscle cannot expand anymore. This can be applied to any muscle group.
