Could either metromine or lovastatin cause muscle cramps?

No, but: Metromine is chlorpheniramine, a systemic antihistamine. It does not cause muscle cramps. Lovastatin, a "statin" (an hmg reductase co-enzyme inhibitor) also doesn't cause muscle cramps but can cause diffuse, flu-like, muscle aching. Lowering the dose often relieves it. A 2 week drug holiday can help determine if it's contributing to muscle pain if you're uncertain.