Could be tendinitis. If it isn't, it may lead to tendinitis at some point.
I experienced the exact same pain recovering from knee surgery and it was due to a atrophied quad. I would suggest some resistance training...squats, leg press, get that quad built up and it took care of my problem (for the most part). If you've recently gotten back on the bike, it could be your base miles have aggravated your tendons due to winter inactivity.
Perhaps a slower base mile buildup? 3 days of 3+ base rides at this time of year is perhaps overdoing it.