Flip Turns = Knee Pain?

Hi, Sorry if this has been discussed already in other threads: My coach has been having me incorporate flip turns more and more into my workouts (I just learned how to do them a few months ago). The last couple of weeks were pretty intense in that I did flip turns for all long distance swims. I am now noticing my knees are really bothering me and it's difficult to walk up and down stairs. So I stopped doing the flip turns so much and my knees are feeling better. Question: have you experienced this same issue? If so, any suggestions for my flip turn form to prevent overtaxing of the knees? Also: do you know of any exercises I can do outside of the pool to strengthen the muscles around my knees so I can go back to doing flip turns all the time? Thanks in advance for your input ...