I've seen beginner's planes in this attitude when they took off, or tried to climb, too fast with too little air speed. Instead of continuing to move forward, the plane just seemed to hang there on the prop for a while. Depending on the plane and the pilot, it could either do as you described, hover nose high, or snap/stall and head for the ground. So, one possibility is that the plane is ok, but that you just tried to force it higher.

However, since you just rebuilt it, it's also possible that something else is wrong. This could include CG, which you said you already checked, the incidence of the wing and/or horizontal stab, or even the angle of the firewall. Before you try flying it again, it would be good to have an experienced person check these things.