Kate Beckinsale continues to leave people alarmed about her as she posted an update from her hospital bed on Easter ... this as her mystery illness drags on.

The actress took to Instagram on Sunday to share a holiday-themed message with her fans ... as she uploaded a pic of just her legs as she wore black PJs with white bow designs and bunny socks.

The pic appeared to be taken from the hospital, prompting several fans and famous friends to ask about KB's health. In fact, Gwen Stefani even wrote in the comments ... "What’s wrong!!!"

Alice Evans sent Kate some well-wishes too, writing ... "I do hope nothing is seriously wrong. I know you’ve been through the ringer in the past year. Sending love and positive vibes."

Kate has yet to explain the reason for her hospitalization, but first sparked attention earlier in March when she shared her U.K. Mother's Day post from what seems to be the same hospital bed. FWIW, we've asked her team what's going on ... and haven't heard back yet.

At the time, she wrote ... "Happy birthday and UK Mother’s Day this last week to my incredible mother. Thank you to those that love us and support us when it’s s*** and try to make sure there are some bits that aren’t."

KB added, "And for looking after our dogs when we can’t, and lead us to remember happy things when we can’t. And turn up when we are sick and sit with us ... And send balloons and check in and pull us out of wells with love."

Get well soon.