Suggest Treatment For Back Pain After Taking Plan-B

Hi,

I had unprotected sex with my husband yesterday evening , I took plan B for the first time in my life since it was my 11day. I had lower back pain last night and this morning for 15 - 20 min and this does seem to be a normal side effect as stated in plan B. Is there any chance for me to gain weight. Please help, let me know what needs to be done next. Thank you