Hmm, mine do that too sometimes. You're right, doesn't hurt just strange! I don't think it is harmful, but I'm not a professional! I've been trying to do more hip stretches and also use my foam roller, maybe try that?!
Hah, I played soccer for 15 years so I feel your pain! It has been helping. I try and squeeze in yoga stretches as my cool down. If you Google hip streches, you should find a lot to try and hopefully work!