Clogged milk duct?

I'm almost positive I'm starting to get/or already have a clogged milk duct. I feel tender on my left breast, on the right side where both my boobs meet.

My son is a month old and I usually have him in the cradle hold when he nurses. I read that clogged ducts usually happen due to baby not emptying the breast as much as possible. When he eats, I only use one side at a time, then use the side he didn't nurse from during the next feeding, to insure he is getting both the foremilk and hindmilk.

My question is, what position should I try to get that spot unclogged? I've been massaging the tender spot while feeding him and also tried the football hold. It seems to work okay, but I don't feel it's very successful and I'm really afraid of it staying like this for too long and ending up with mastitis. Can that even happen?