What Causes Scanty Periods?

So I had a tampon in today but it is the first day of my period, so my flow is really light. It felt like it might be leaking (only because I felt fluid in my underwear) so I went to take it out and it wasn t that dirty when I pulled it out but my underwear was very wet, but clearly not with blood. When I smelled it, it was clearly urine. I didn t not feel at all like I had peed. Why did this happen?