The British comedian - who proposed to Katy just three months after they met last year - claims he experienced a feeling similar to "concussion" when he and the 'California Girls' singer went on their first date.
He said: "I immediately knew that it was either concussion or love. After that we went on a date and I was anxious and nervous on the date. Normally like on the first date I'm alright because it's like it used to be to me... the only date. But this one I felt all anxious like my mind was awash with hallucinogens - peculiar! I had to hold on to surfaces.
"I thought it was either something wrong with my starter or love. So now we can narrow it down to concussion or food poisoning or love."
The 34-year-old actor, famed for his womanising ways, also revealed his mother Barbara was pleased he finally got engaged because she didn't think he would ever walk down the aisle.
He added in an interview on 'The Tonight Show with Jay Leno': "My mum was beginning to wonder if I would ever become married unless it was in some kind of perhaps, Mormon style group wedding, to perhaps a group of cheerleaders.
"We went back to England to my mum's house in Essex. She was very well behaved and helped with the dishes. There was a bit where my mum spilt some gravy and I thought, 'That's good isn't it? Normal down to earth conduct.' "