Welcome to the forum, Yaniv.
The glans is sensitive (kind of the point with it, I guess) and if you stretch or jelq while pressing hard on the CS (the soft part underneath your shaft), the blood will be pressed hard up into the glans. You probably just used too much pressure. Try different grips (overhand, underhand etc) to put pressure on the side chambers (CC:s) and not so intensely on the underside of your penis. The dots you saw were most likely burst bloodvessels.
Just try to change your grip, and get the pressure in the CCs, not the CS.
regards, mgus Taped onto the dashboard of a car at a junkyard, I once found the following: "Good judgement comes from experience. Experience comes from bad judgement." The car was crashed.
Primary goal: To have an EQ above average (i.e. streetsmart, compassionate about life and happy) Secondary goal: to make an anagram of my signature denoting how I feel about my gains