He teases, am I over sensitive?

My boyfriend would sometimes tease me, not really in an intentionally hurting way, like he would tease about things I did in the past that didn't work out the way I envisioned, or say stuff like "come on, let's go, sandbag" when we go run or bike together. I have explained to him a few times, but his reaction has normally been "don't think too much of it", "don't be too over-sensitive". We just got off a bad conversation beginning with him "joking" about something I did in the past, and it turns south after the "hurting" "over reacting" thing starts, saying why do I hold stuff in the past to go against him. I have full intention to discuss the matter but he just wants to leave it to later and the discussion ends that way. I'm okay leaving space and time for him to ponder. But am I being "too sensitive" to some of those "jokes"? If I feel it and express it, and he doesn't remember, what to do when it happens again? Is it not a healthy way to bring up stuffs happened in the past?