Sudden loose bowels and diarhea(diarrhea) very uncommon for me

A little over 2 weeks ago I woke up to severe cramping followed by a timely bowel movement that was loose at first and ended as diarhea (diarrhea). Since then I will have 3 or more loose bowel movements for a few days and then have a day or two of nothing. Last week I had diarhea (diarrhea) so bad I was going every 10 minutes to the point nothing was coming out but just cramping and pushing. I called the docter and they gave me an antibiotic and diarhea (diarrhea) meds said I had a bug.....No bowel movement for 3 days and finally had a soft bowel movement, the next day 2 and then 3 loose bowels movents the day after that..then a day of nothing, yesterday I felt like the first day it started  bad cramping and on the toilet for 30 minutes like its starting all over..... I  quit smoking about 4 weeks ago taking welbutrin. I also had been eating Activia yogurt because normally I only have a bm every other day and is somewhat hard, thought this would help, Ive eaten this before without this reaction and for so long, doesnt seem to be why Im still doing this..Any other reason this is happening