Nipple discharge?

I'm 34 and I get clear nipple discharge from time to time, just a small amount that I can tell because I have a small wet spot on my shirt when I'm not wearing a bra. It doesn't have a color or smell. The breast this happens from does have a, I wouldn't say lump but it feels like a swollen area in the breast, like the glands or something, it's tender. I only notice this when I'm not on birth control pills. When I'm on the pills I don't feel it or have the nipple discharge. I had my pap in October and my doctor checked both breasts and didn't feel anything or didn't notice it. What could it be? It's been like this for years, at least 6 or so. I've had several doctors checks my breasts and they never noticed it or if they did they didn't say anything. But like I said, I never notice it on the pill, only off the pill. I'm not pregnant or breastfeeding btw.