Having Brown Discharge, Missed Periods, Stomach Thickening. Chances Of Pregnancy?

I just turned 40 in October, am fit, look young, and have a small youthful frame. I smoke about 8 cigarretes a day and have been taking anti-anxity/panic attack medication for the past several years (I very, very rarely drink alcohol). I think I may be pregnant! I had a period at the beginning of January (the 2nd), but at the beginning of February there was just a minimal brownish discharge for a few days (no requirement for hygiene products or anything). I heard that when an egg is implanted the discharge often looks as I ve described. It is now March 8 and no sign of a period yet. Plus I ve increased my workout routines during the past few weeks and actually feel thicker around the middle. Could I be pregnent? Help please.