Errrr... Are you sure this is not plain old *hunger*? I mean, when hunger
gets intense enough (and you're not in ketosis), it can get a bit painful,
or create stress that cause psychosomatic pains... Did you cut your calories
a lot while switching to your new diet? Usually, people recognize hunger as
such, but some are quite unable to do so. It seems we can experience
specific hunger (usually with specific cravings) when we haven't eaten
enough proteins (actually, some authors claim it can be amino-acid
specific). Beans are a decent source of proteins, though not as dense as
meat.
How did you design your vegeratarian diet? Do you still eat cheese, milk,
eggs? What did you replace meat with? The more you remove animal products,
the more careful you have to be with designing your diet, because you have
to build your own complete proteins from the uncomplete proteins you find in
vegetables. That's where you could benefits from the advice of a long term
vegetarian, a dietitian or a nutritionist...

Since I don't know the detail of your body status and your diet, I can
only make a guess. There's one possible reason that you probably ate
too much fiber. Too much fiber would cause a faster intestine and colon
peristalsis. That may, sometimes, make you feel painful around the area
of belly button.
it's only a guess. I feel that you don't have a nutritionist to guide
you through this.  I suggest you should ask one for some advice or
you'd better read some book about dietary.