Draw an imaginary line down the middle of your breast and then across the middle, making a cross with the intersecting point being your nipple. Find the lump within that space. If it is on the top left of your left breast or the top right of your right breast, call right now to make an appointment. Call this week anyway, and they will have you do the same thing, likely. You can have an ultrasound done on the breast to see if the lump is fluid filled or solid. It is common for nursing moms to get fluid filled lumps that are not milk. These are totally not a big deal and nothing to worry about. Having a needle aspiration is a bad idea if you are still nursing because the needle can puncture the milk ducts. I would not wait another 6 weeks. Why wait when you can have some peace of mind now? The breast u/s takes only a minute and generally shows a fluid filled lump. Voila, you are done and no more worries.
Hope that helps!