Hi, there. I'm sorry you're suffering for your art. My hands hurt more when I knitted tighter and my arms got sore too. If your stitches are tight then you have to work harder to knit them and it puts more stress on your hands. You can try knitting looser. For a long time just trying to knit English (I'm a Continental knitter, I hold the yarn in my left hand) made my hands hurt and I'd really tense up all over. I find that washing a sinkful of dishes in water about as hot as I can stand helps my hands. The movements and motions I use to do the dishes while my hands are in the water makes a big difference, probably something like Antares in the shower.
Yeah, don't stress over finishing by the shower. She's your BFF and will love that you're making something for her baby. She won't want you in horrible pain just to be done by the shower.
One more thing: BREATHE. When I get focused and involved in something like learning knitting stitches and patterns I do forget to breathe, do you?
I should never overestimate my basic arithmetic skills.