sounds like a clogged duct. Watch for signs of infection- if it gets hot to the touch, red, inflamed, etc... you may need antibiotics. You may also get cold/flu symptoms.Since it has been 15 days, I would contact your OB. Chances are you have mastitis.
There are several suggestions out there for getting rid of clogs. Warm compresses can help. Have the baby nurse in different positions. Lots of nursing on that side. Doing compressions while you express. (Hopefully the other ladies can give you more suggestions)
What i found really helped me was using the manual pump that came as a freebie with the expensive medelas. I also got the same one free from the hospital. It looks like a bicycle pump. While it is not good for expression on a regular basis, i was able to adjust and control it to do long, hard s*ucks that removed my clogs within minutes.
When my sister had a clogged duct she used a vibrating "massager" on her breast - the ones used for back and neck massages.
I have the same problem now but not as severe as you yet as it went away with a cold compress and hand massaging. But my midwife told me next time it happens get the baby to suck so that its lower lip/chin is positioned on the upper left region. She said this is done by laying the baby on its back on the floor and strandling it with your head towards its belly - kinda like a milk cow. I have not tried this yet but if you do let me know if it helps :-)