When you experience anxiety whatever the reason it can become an expected feeling. That's how we end up with chronic anxiety -- we get anxious in one situation and start to expect it in others. This could be what's happening to you. Increased anxiety is one of the most common side effects of antidepressants, as they can be stimulating for certain individuals. Some are known to be more stimulating than others and lexapro isn't one of those, but it can happen with any of them. As for gastritis, that isn't really a diagnosis, it's a description. If it was caused by the Lexapro, it should all go away unless your way of thinking about the experience causes you to hold onto it. Have you ever looked into therapy?
Contact the medical provider that issued you the prescription for Lexapro. Your doctor saw the need for this medication and needs to be informed of its side effects so that something else may be prescribed for your benefit.