Is it possible that a change in seats can cause this issue?

The old plastic seat on my Raleigh died. The cover came off and I rode home on the steel frame a few weeks back (don't try this, you won't like the bumps in the road). I had a NOS Raleigh seat and put it on. It seemed too small so I added a gel cover. Sometime since then I have begun having heavy leg cramps and sore knees but didn't think of the seat. Last night at the bike coop I was having trouble bending if I dropped a part and one of the other riders said immediately, your seat needs adjustment if your having pain.

I only do 20km (roughly 12 miles) a day and on a flat route) and usually the only thing that bothered me was summer heat.

Could this guy be right and I need a seat adjustment somehow or even a new seat????