He wasn't at 100% power when he made the Cell Jrs, was he? So maybe he was putting out ki at 70% of the rate possible and then used some of the other 30% to make the Cell Jrs. The higher ki use could have weakened his later power up vs SSJ2 Gohan. 70% is just the example number.
Though I'd expect his raised ki to be detected by the onlookers if he used some ki to make the Cell Jrs. So it could be he dropped his ki to his body from 70% to x% and used that (70-x)% to make the Cell Jrs. This way the ki detected from Cell by the Z fighters wouldn't change, still he's still producing the same amount of ki. And it wouldn't tire him out any more than if he was using the 70% to strengthen his body during that time. If Gohan had punched him while this was happening, it would have hurt a lot, but the shift in ki use wasn't detectable to Gohan or the others. It's similar to how no character ever mentioned detecting a difference in the ki of Piccolo or Cell when they were redirecting their power to regenerate (it drops their ki afterwards, but no onlooker ever mentioned feeling a difference in the nature of their ki as they get ready to regenerate). I'll go with this one.