Sounds to me like your hip flexor muscle. My guess is that you need to take some time to stretch that out before you begin bowling. Sit with your left leg crossed over your right with your ankle on your right knee. Press down on the inside of your left knee gently stretching this important muscle out. Hold for 10 seconds and repeat two or three times. There are other stretches that will help as well and you can look them up online. This one works for me.
Jay
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