I am pregnant with twins too. I am 21 weeks today and I too have been having horrible contractions and pains =( I go for my next ultrasound on the 3rd to check my cervix, my cervical cerclage, and my babies. I suffered two previous second trimester losses one at 20 weeks and one at 24 so I am a nervous wreck!
With this pregnancy I feel a lot of pain and pressure just like you described like I need to go number two and nothing is happening. I just hope and pray my babies are ok and will stay inside me until they can survive outside of me!
Keep us updated if you find anything out!
You're probably experiencing Braxton Hicks Contractions which are common after 20 weeks. As long as your doctor says everything is okay and you're low risk, then it's totally normal. Some women will experience these contractions and some won't at all. It's just your body getting ready for the real deal. Sometimes the pains mean you're dehydrated or that you're putting too much pressure on your body. Just keep your doctor informed of these symptoms at all times, drink plenty of water and get rest:)
Congrats on your twins! :) You could have your doctor check you're cervical length, and for any funneling of the cervix that could be causing the pains....maybe they are just gas pains??