Funny thing happens to my voice when I sing above Tenor C Is this bad?

I've only been in vocal training for 3 weeks. And I can hit Tenor C in full voice without difficulty (other than minor muscle strain) on a good day I can make it to the D# above Tenor C, but it's very hard not keep my diaphragm from losing it. When I try for E, and F. My voice sounds like a falsetto, even though I'm not using my falsetto. It's vary hard to make this happen, and it's almost like I have to NOT think about this. If that makes any sense.