Can not attribute the constipation to either medication. Antibiotics most commonly cause diarrhea, not the reverse. Stopping the Alli may have caused a change in your digestion, as increased bowel movements are associated with alli use. You should be regular again after a couple of days.
Alli does not flush out medications or vitamins. It works by inhibiting the fat digesting enzyme lipase, which will then block the absorption of fat. Any thing that requires fat to be absorbed into the blood stream can then be affected( by decreased absorption), while Alli is active. If you have any questions regarding medication use, do ask your pharmacist or physician.
Hope you are feeling better!
Sophia De Monte RPh