hallucinations

Why am I having hallucinations?

Yupp, I did not type wrongly.

H-A-L-L-U-C-I-N-A-T-I-O-N-S!

It started when a few days ago, I thought I heard my friend calling me in a bus. When I turned around, I didn't even see anyone I knew.

Then just now, I thought I heard someone knocking the door twice. When I went to the door almost instantly, there wasn't anyone there. Nobody. Even my neighbour's cat wasn't there.

Then, I also heard my phone sms ringtone. By the way, my sis and mum was not at home. My dad was working. When I went to check my handphone, I did not receive any messages.

I was certain that I did hear something.