Especially when taking meds, that is very common. Depending on when you take them, the effect they're having, in addition to a slew of general, daily variations like time of day, blood sugar, and so on. Maybe try to discover what conditions are most conducive for your highest energy levels and try to schedule exercise around the times that seem to be best. If there is no noticeable pattern in your particular case, which sometimes happens, then working through it is about all you can do. I know stimulants are out of the question, but there is a drink powder called Emer'gen-C that one of my hypertensive clients swears by for some extra energy once in a while (and I started using it too
). Just B vitamins, C and minerals. Might help ya out. Good luck.