It may be late-onset OHSS. I'm experiencing the same - tummy distension, breathlessness when walking fast or climbing stairs. My first scan 2 days ago showed my ovaries are still enlarged and there's fluid in tummy. Thankfully I don't need to be hospitalized, just continue monitoring on own.
Unfortunately in cases of pregnancy this will last for a couple of weeks, may be up to end of first trimester. Maintain a high-fluid (non-carbonated isotonics are good), high protein diet. Take things slow. As h0peful mentioned, if tummy grows bigger (try to measure your tummy girth everyday) and there's weight increase everyday, accompanied by worsening discomfort, please contact your IVF centre.