Can stress cause your stomach to feel knotted up?

Stress can cause all sorts of strange symptoms in the body and tension in the stomach area could definitely be one. 

As well as taking steps to relieve the stress you could try Passiflora complex which is traditionally used to ease nervous tension and anxiety. 

It can take a week or two to feel the effects so if you've not felt any improvement after a month it is worth trying an alternative until you find what works best for you.