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.