That irregular bleeding and spotting sounds more like due to the hormonal changes that can occur at age 37. It's likely due to not ovulating rather than being pregnant, since your pregnancy test is negative.
I would recommend that you see your doctor for evaluation. A blood test can confirm whether you're pregnant or not. If not, which is most likely, then some progesterone pills will help to clear up the spotting. Best wishes.
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It's very common for people to gain weight after they stop smoking. Usually that can be helped by daily exercise and watching what you eat.
Part of the weight gain a larger abdomen may be due to hormonal changes and the resultant water retention. That part will resolve once your hormones are regulated.