I'm 24 weeks today and I've been having a lot of pressure down in my lower abdomin as well - especially when I sit for an extended amount of time (like at work). I'm pretty sure it's just the baby sitting low and/or your body adjusting to all of the changes. I make sure I drink a lot of water, try to walk around as much as I can throughout the day and when I get home I just plop my butt on the couch and put my feet up. I wouldn't be too concerned unless you were experiencing other symptoms - which you said you aren't - so in the meantime just enjoy knowing that it's your LO growing/gaining weight 
