She has obviously done it on purpose to try and shut you out.. probably jealous that you are having a baby with him now so shes not the only mother to his child..
I know some people might disagree with me here.. but this is what I think you should do.....
Talk to OH.. tell him that the best plan of action is for him to politely decline the offer to go to the party.. BUT!!! before anyone jumps down my throat here.. the reason for this is.. the little boy in the middle of all of this is 3.. he is at a very impressionable age.. and he will KNOW that Mummy and Daddy's girlfriend dont like each other and he will eventually start feeling negative when it comes to parties and times when your OH and his ex are together.
Your OH and his ex need to separate their lives now, which includes separating the lives they both have with the child of theirs. The 3 year old needs to understand that Mummy and Daddy no longer spend time together because they dont live together anymore, but Daddy still loves him very much and any birthdays will be celebrated separately. So on his birthday, he will have a party at his home but the next day, Daddy and his girlfriend will take him out somewhere special.
Children remember less about what they did on their birthdays and more about the atmosphere around them. If this little boy sees that you are being excluded he will expect you to be excluded from everything because he will think that is normal. You and OH need to find a way of including you in everything.. which means having to separate contact between Mummy and Daddy.
That's what i would do anyway.. I understand that alot of people might think the best way is to let your OH go round there for the party and you just have to put up with feeling excluded but I strongly disagree. You are pregnant with his child which means you need to be included.. and by having your OH go round there without you, the poor little guy might get confused. And as time goes on and this continues, it will get worse! It's his birthday.. so.. party at home.. and then Daddys house the next day and Daddy has promised we'll do something really cool like go to an indoor soft play area and then McDonalds or ice-cream afterwards... and then we'll go to the toy shop and pick out a brand new toy!! And guess what? Daddys girlfriend is there with us and it's happy and there is no animosity and then I get to go home with my brand new toy and see Mummy and tell Mummy how great my day was with Daddy!
Sorry for the long post.. but I had this problem when I first got with my OH. The only difference was that I wasnt pregnant.. but I wasnt about to allow her to exclude me.. I was as polite as I could be to the ex, and more mature than her by a mile (even though she is more than 10 years older than me).. and now I get on with my OHs kids really well. The boundaries have been set and the kids are happy and no one feels excluded!
Best of luck hun
xxx