My question for you is what variety of seeds are you using and where did you purchase them?

Your plant sounds like it has tendencies of a bush variety pumpkin...they don't vine out but grow more upright and get an abundance of leaves on the top.

It is the Atlantic Giant and I purchased the seed packet from Thompson & Morgan online

Hmmmm...well then I am stumped too. We grow lots of pumpkins and my husband is growing Atlantic Giants....putting a lot of effort and time and fish emulsion/seaweed into his. His big plant has about 15 huge leaves and one really good vine extending out
But he started it pretty late...like 4 wks ago..due to weather.
Your plant....being a couple months old....should be vining...especially if it's flowering.

Got a photo?

I'm guessing you have a flatvine/ribbonvine. I've had a couple of these in the past. They occur occasionally, as do doublevines. Are you able to see any small normal looking vines coming out of the plant? If so, I'd train one of these as the mainvine, and prune off the growing tip of the flatvine. This will hopefully focus the plant's energy into the normal vine.

I have a different sort of strange pumpkin plant. The sepals on the flowers are not the normal needle-like shape, but instead have been replaced with what look a lot more like leaves. It's very odd. Also, the stem and petioles are mottled. Has anyone ever seen anything like this?

It came from a packet that are supposed to be Atlantic Giant. I got them as a freebie at a giant pumpkin growing seminar. The speaker who distributed them said they were a gift from Howard Dill.

I do have photos but I don't know how to post them.