I'm on 300 mg of ranitidine and it actually helps me more than my additional prescription of 40 mg of Nexium. I did have some troubles with feeling weak shortly after I started taking both, yet had not had any such trouble when only on Nexium in January-February.
From what I've read, these drugs can interfere with absorption of iron, as well as other nutrients, including vitamin B12. Both of those a big issue energywise for me, so I now take supplements every day when my medication cycles are at their lowest (usually breakfast).
Bring it up with your doctor for sure, since it's seemed to make you feel bad. I think pepcid and ranitidine are in the same class of drugs (H2 histamine blockers), but not sure.