Why do i have a cramp?
There are a number of reasons you could be cramping - if that is, in fact, what it is.


You are not properly warmed up before exerting yourself
You are exceeding your limit of fitness
You are not properly hydrated
You have not eaten enough food to fuel your ride


Without additional information it's really hard to diagnose something like this, but those are some common causes of cramping during a ride or any athletic activity. You may also look at your technique to see if you are favoring one side over another or have a setup that is not ideal.