You soon found yourselves changing somehow. There didn't seem to be a trigger with it, no gas, no rays, no nothing, but it was obvious both of you were changing somehow. And whatever it was, you and your girlfriend seemed to be changing into the same thing.
You both became significantly smaller, at half your original size or more. Your skin greyed out, your limbs and digits changed, your heads lost their head and stretched this way and that. Your senses altered a bit, most notably your with your sight. It was obvious partway through the transformations that you and your girlfriend were turning into the grey aliens in the movie, but a combination of fairly fast transformation and your now very large-and-unchanged clothing was covering up some of the details. Soon the transformation ended, and you each took off your clothes to really take in your new forms.
With both a combination of a nearby mirror in the room and having identical appearances, it was easy to see how you both looked like. With no real frame of reference other then your clothes and now-matching furniture around the room, you would imagine yourselves around 3 feet tall at an estimate, with a good third of that going into your new heads. You had a new grey skin, wrinkled and smoothed out perfectly of being an alien with not a hair in sight. You had short legs but long arms that went almost to your knees. You had 3 fingers on your hands, luckily with a thumb, but they had odd lengths at each section with no nails and "bulbs" of flesh at the ends. You have two toes on your large feet spread out in a "Y". Your torso was probably the most "human" out of your body, looking more like an emaciated human's with a some what prevalent gut. Overall you were a kind of gangly looking if it wasn't for your gut. You also notice you and your girlfriend had no sexual characteristics to speak of, including genitals or nipples. Did you even have genders at this point?
Your heads were those most exotic parts of you. Very large, black orblike eyes leaning towards almond shaped and featureless. Very small slits for a nose, a small, lipless mouth that would be easily missed if it wasn't for movement. No ears to speak of, but from your motions and ambience of the ship so far, you did seem to be able to hear somehow. The top of your heads were huge bulbous things, stretching up and slightly back. Your heads weren't just for show, were they?
"Jesus, what are we?" your girlfriend said, asking the questioned you both seemed to have. It made both of you shiver as it wasn't her own voice and after some testing, your voices were also identical. You were now grey aliens obviously, but your new bodies were ALIEN in more ways than one.