Knee Swelling

Last night, a couple hours after a 5 mile run I noticed that there was a descent amount of swelling above and below my knees.  It doesn't hurt at all.  It is just squishy like its fluid filled.  I experience a little bit of knee pain during runs, but it never lasts the whole time and there is no residual pain when the run is over.  Is this swelling something that I should be concerned about?  I guess my questions are why is it there?  And, can I continue to run without damaging anything?  Anyone that has had this or knows about it.  Please chime in.