Has anyone else had to stop feeding due to medical reasons against their desire?

I did notice my sons buddha belly kinda went away around 3 months of age, but he was still showing signs he was getting enough milk and the dr. wasnt too concerned. but at the 7 month checkup they werent happy with his weight gain because he was in the 2 percentile. So I started giving him formula, but still nursed him upon waking, throughout the day and before bedtime. I saw a major improvement. Hate to sound like a "bad mommy: but when he was 3 months old I picked up the bad habit of smoking ciggies again (only when he napped or was in bed for the night), and I did some research recently, which showed nicotine can reduce the fat supply in milk. I think thats what happened with me. He self-weaned off me 3 days after his 1st bday and I feel blessed he still fed somewhat ped my "goal", even though I wish he was exclusively fed his whole first year. But the formula was very helpful and he still got nutrition from me (my ies that is....) first!