I hate to sound like a broken record but please please please never go under your truck again with just a jack supporting it, let alone the factory emergency jack!
The emergency jack is for just that, emergencies...they are very easy to topple over. When I was younger and dumber I've had a car fall off emergency jack while I was laying on the ground. Fortunately for me it toppled away from me and not on top of me.
But even if you had used a bigger floor jack still never go under there without proper jack stands supporting it. Just make it a habit to treat any jack as if it could give out at any second and you will stay much safer
Not sure if you are new to wrenching on cars/trucks but if you are, once you get yourself a set of jack stands, read the factory manual to see where the proper support points are along the under carriage and set them under there. Finally, what I like to do is give them a quick "kick test" before going under to make sure it's all nice and sound.