Sandi, this may help bring you some peace about this particular situation or others that are outside of your control. See, you DID tell your husband you didn't really desire to go, but apparently the right thing WAS to go, so you went.
Even though it doesn't change the tiredness, I believe if you shift your focus from the tiredness, sickness, and not getting out of going over there that you will FEEL an increase in energy, more peace and even a bit of happiness that it turned out this way.
I can just imagine how wonderful it was for this this elderly relative to have you there and get to serve a special meal. Even if the elderly folks complain and fuss about our help in the kitchen, they love the companionship, and treasure the memory on other lonely days.
You weren't doing something to neglect your child, your husband was there and available. Not knowing all the family dynamics I'll just venture to say that when you honor your husband's birth family and enjoy them that it's blessing him as well. That's Biblical wisdom- to bless your husband.

Unfortunately families often treat each other like the rest are there to put up with whatever each individual decides to be, say, or do. It's impossible to change others, so the only way to stop feeling imposed upon is to voluntarily give more than they ask when you do interact.
I whole heartedly agree with letting dh say/do what he wants to do. Sometimes I do have an opinion and he asks for it. If I don't know what is the right thing, or he doesn't want to know what I think, it sure is nice to just let him make the decision and never think about what could have been otherwise.

Then tired is just "tired" and I go home and rest and if necessary make things a little bit more simple at home while I recoup, but there is no confusion or guilt that it happened the way it did.
Often I think it is not so much having the wisdom to make a plan ahead of time or pick the best plan in the moment, as it is choosing an God honoring attitude.
........I'm the nutritionist junkie who likes telling people what they can do to feel better, and I'm holding in suggestions to help you over this postpartum hump.

Hope you feel better soon.