What Causes Pain In Breast 8 Days After Taking Next Choice Pills?

I'm 29... had unprotected sex (14 days ago)... don't want another baby yet so I took Next Choice (morning after pill) 12 days ago for the first time in my life... anyhow... didn't have a whole lot of side effects except exhausted but that could be from lack of sleep... anyways breast pain started 8 days after taking pill and has not let up for the last 4 days. How long is this side effect suppose to last and why the side effect 8 days later? I'm completely stumped... can't stand it anymore when can I expect to get back to normal?