Welcome to Crochetville!
Hdc is a fairly thick stitch. You're using very thick yarn. That means you need a huge hook. What happened is that you have too much yarn in too little of space.
Also, magic's suggestion of counting your stitches now is spot on. If your stitch count isn't still 40, your sides will not be straight.
As far as fixing, the only way to fix it is to rip it out and start over. I highly recommend only using sc or extended sc with that yarn. Before you begin the blanket again, make a swatch 10 stitches across by 10 rows. Check that EVERY row is 10 stitches. Check that it lays flat. If yes, make the blanket. If no, make adjustments, like a different hook size, and try again.
Let us know how you do.