1. Everybody reacts differently, we all have different body chemistry. There are some lucky people who never experience major side-effects, but almost everyone experiences some occasionally.
2. The lower doses (2.5-7.5mg) are frequently called "loading" or "conditioning" doses and are intended to allow our bodies to adapt to the medication and any side-effects gradually.
3. While there isn't any solid science behind it that I'm aware of, anecdotal reports suggest that injection location can impact the degree of side-effects experienced. Some people report feeling stronger side-effects injecting in their abdomen and weaker injecting in their thigh or arm, or vice-versa lol. Again, everyone's a little different.
You could just be one of those people who won't start feeling anything until reaching a higher dosage step. Oddly, more energy is not something most people report, in fact a lot of people report experiencing occasional fatigue after the first few doses.
I felt side-effects about 2 hours after my first dose. For me it was a fullness feeling in my stomach and a tiny bit of occasional nausea. I found the nausea was usually triggered if I ate anything greasy or fatty. Meclizine worked great to help the nausea. After about 2 months I never really experienced any more nausea unless I let myself get a little dehydrated or forgot to eat (it happens). The fullness feeling was intermittent; some weeks it was really strong, and then I could go a month without feeling it and randomly after an injection it would come back strong again. That's probably one of the biggest things for people to know; how the stuff makes you feel can vary dramatically week-to-week, day-to-day, and dose-to-dose. A lot of people are too quick to assume something's wrong when that happens and start making some bad assumptions about the med or themselves that aren't true.
I'm curious where you injected, and suggest you try a different location next time to see how your body reacts.