It is poop. You need to poop girl. Its all backed up in there because you haven't gone for three days. The poop just keeps building up until it bulges into the vaginal wall. Go to the toilet and push all that poop out. See if you can still feel the lump. Pressing on the lump to help guide it out is good too. You need to take a poo though. Three days is too much. You'll be very constipated and it won't come out easily.
I guess it could be, but it does seem a little odd that you could feel something squishy through the walls of the vagina, since they are pretty muscular. I could see you being able to feel it if it were a solid mass (like impacted stool), but more squishy? Sounds more like you are just feeling the normal tissue there.
While at the beach, pick up some Benefiber or Metamucil and take it, and see how the poops do.