Not Invited; Should I go?

[Precious P]

A friend of mine from a former job is having an engagement party soon and I have not received an invitation as of yet. She's invited everyone (five of us) of the group from our former job that we are friends with except me. We did not have a falling out and from what I know everything was fine between us (we do not talk on regular basis, but then again most of us don't; we catch up on Facebook, texts, and the occasional outing).

A friend of ours has said that she will take me as her +1 but I feel that if I didn't get an invitation that I shouldn't go. I am sure that it was an oversight and she didn't mean not to send me an invite but I also feel that if she really wanted me there, she would have sent the invitation. The friend then told me that I was being dramatic, of course she wanted to invite me and she probably just forgot in all of the hoopla (but managed to remember the other co-workers). Would it be wrong if I declined to go? Or should I just suck it up and take our other friend up on the invite and just brush it off?
Reading these answers, pretty much confirms my thoughts on going (our other friend said 'honestly, you're making a big deal out of nothing; you know she probably just made a mistake and you should go with me'). I still feel very confused because why wouldn't I be invited? I've been invited to other parties and get-togethers she's had and I've always gone. I've even gone through old messages and texts to see if I'd maybe said something offense or off-putting and I can't see anything that would give me the impression she was upset (I know, I have to try and rationalize things). In fact, the last conversation we had is when I congratulated her on the engagement about two months ago. I really feel it was an oversight but I feel a bit hurt that it would happen since she's always invited me to things. Part of me wants to ask but I don't want to guilt her into an invitation or seem like I'm asking to come.