Quads are sore?

I did a a team triathlon this past weekend on Saturday. I did the swimming and the running. I went and swam on Monday, and before I went swimming my quads were already sore. They have never been sore from working out like this. I did play basketball monday and tuesday as it is another hobbie, but i played basketball in college and my quads were never sore, it was more of my hamstrings so I know that is not the problem. Am I swimming to much with my legs or just do not have the strength yet. The triathlon was only a 325 yard swim,  and the run was a 5K.  I only swam about 1000 Meters on Monday. I usuallyswim at least 500-1000 meters at a time, so I don't know why exactly my quads are so sore.