You could have pinholes. Socks take a lot of abuse. To check for pinholes, fill the suit up with water- not all the way- just to say, mid calf. Check for any leaks. Aquaseal if you find any.
If no pinholes, you may just be dealing with physics. Water vapor is going to travel from high vapor pressure to low vapor pressure. Vapor pressure is dictated mainly by temperature, that is, higher temps tend to produce higher vapor pressure. So, your waterproof/breathable material will let much of your sweat out, however, at colder areas in your suit, like your feet, or your butt if you're sitting in a puddle of cold water, you will have elevated humidity and condensation.
I wear extra socks, and try not to step in the water, even though I'm wearing gore-tex socks.