I'm kind of experiencing the same thing. Getting bigger and bigger (also due in May), but not too keen on spending a lot of money on maternity clothes.
I splurged for a couple pairs of good fitting maternity jeans. Other than that, I wear long sleeve shirts from my pre-pregnancy wardrobe. When I worry they won't cover my whole belly, I layer with one of those hip-length long layering tanks or a bella band. You can wear those tanks during your non-pregnancy days too, of course. :) And the final piece of the wardrobe, IMO, is a few of these really awesome sweater coats that are in style these days. You know, the long sweaters that belt and go down past your hips. They go great over a pregnant belly, they're super warm, and if you are worried about unbuttoned pre-pregancy jeans slipping a bit too low, the sweater coats cover all that up.