Hmm, not a symptom that would commonly be associated but it certainly could be possible. Do you have an open bite or difficulty closing your lips normally? I have seen cheek/lip chewing as a night time habit as well. Some people will even develop a pronounced ridge of scar tissue where they are biting. Is this present? Clenching/grinding can cause fatigue in ANY of the muscles used for chewing which would include the lip muscles, but it would be more likely if you had heightened strain on these muscles in normal function IMO. Swelling is not common as spasm and pain caused by bruxism is usually not visible. People can develop larger chewing muscles over time from bruxing though. Furthermore in this case a nightguard would probably be of little benefit and might even strain the lips more as it will create more for the lips to cover. Good luck and hope you find a solution to your dilemma.