What does pinching your knee cap mean?

My boyfriend and me went skating last night, and about o half way through, it happens. Now keep in mind, I'm probably an idiot for not knowing what this means, and also my best friend and me were in band seventh period, and she told him i was ticklish, on the bottom part of my legs (which i am), but anyways back to the point. We ended up sitting down, and I pulled out my phone and started txting my other best friend, when I was half way done txting the sentence, I felt little, gentle, pinches on my kneecap. I looked up from my phone and my boyfriend was bent over gently pinching my kneecap. When I looked him in the eyes, he shot my a wicked smile, and then I thought nothing of it and went back to txting. The reason I mentioned the ticklish thing is cuz I think my best friend told him as a joke. Please help! What does that mean???