I resent stop taking nexuim because I've had gas at bottom belly but I still have gas sometimes what else could be causing it and should I be worried?

Wasn't Nexium. Nexium influences the rate of acid production by stomach glands. It has nothing to do with your gas production. You swallow air when you eat & germs in your gut create gas when they feed on debris. Some foods like cauliflower, beans, etc. make people gassy because the gut germs feed on their debris. Beano is an OTC product that can help. Keeping a food diary to identify what to avoid also helps.
Gas. Many foods are gas producers—beans, cruciferous vegetables (broccoli, cauliflower, cabbage) melons & cucumbers (some people). People with lactase deficiency (the enzyme that breaks down milk sugar lactose) can develop gas & diarrhea whenever they eat dairy products. Otc simethicone &/or beano can help break up gas to ease discomfort. Lactase tablets, lactose-treated dairy products are available.