Did you have a sinus lift procedure with the implant? The sinus membrane is so thin that even though it looks as though the implant is in the sinus it is possible that the membrane is intact. If your insurance will cover it I would seek out an oral surgeon associated with a teaching hospital for a second opinion. They should do the CT for a clear picture of what’s going on. I just had a dental implant with sinus lift that protrudes into the sinus but doesn’t cross the membrane. An oral surgeon I later visited for a wisdom tooth extraction said I was lucky the procedure worked without complications. He said he spends a lot of time fixing these types of implants when complications occur. Good luck!