YANBU - it will not be "lovely" as somebody suggested, if you have a strained relationship!
I travelled 4 weeks after DC3, but on my own terms - don't let anyone guilt you into travelling soon after delivering your DC1. You have no idea whether you will go overdue and only be 3 weeks post birth, have a c-section, be struggling to BF, be sore from a tear, have a bay who never sleeps for more than 45 mins and be exhausted - or whether for you it will be a breeze with an easy delivery 3 days early and an easy, contented baby, and you will be full of the joys of spring by Christmas, but nobody can tell you that their experience will be yours - just because one person flew single handed across the ocean whilst breast feeding their 2 week old and writing a block buster novel, doesn't mean the next person can manage a 2 hour train journey comfortably.
There is much eye rolling at "Mummy Martyrs" on MN sometimes - but this seems to be at term directed exclusively at those who adapt their lives around their children, usually because doing so actually makes their own lives easier - apparently mothers of babies and young children are eminently U in the eyes of some if they don't put themselves to considerable inconvenience, discomfort and stress to fit in with the whims, wishes, quirks, and expectations of adult extended family, friends and vague acquaintances... The double standards are somewhat breath taking sometimes.
Say no to your mum, you have no way of knowing if you'll be up to it, whether you'll feel energised and want to show your baby off, or cozy and loved up - or exhausted and uncomfortable - and want to stay home in your sloppy comfy joggers.