At around 6pm on Thursday a man gained entry to the pensioner's home and took her into the kitchen where he pretended to check the water.
Meanwhile, several other people entered the address in Rowley Regis and made off with about £30,000 made up of old banknotes.
It happened just one day after a woman, also aged 90, was dragged out of bed and robbed at her home in Walsall. Police said there is nothing to suggest the two incidents are linked but stressed the investigation is in its early stages.
Detective Inspector Phil Griffiths, from West Midlands Police CID, said: “We are determined to find who is behind this burglary that has understandably left this poor lady shaken.
“Something I want to highlight is that a large amount of the money stolen is made up of old banknotes that would have to be changed for the new notes at banks.
"We are working with banks across the region to make sure this message is cascaded out to all branches.
“If anyone is asked to change old notes for new ones, I would urge them to contact police immediately.
“We’ve carried out forensic examinations, taken statements and are examining CCTV.
“Patrols have been stepped up in the area and we are continuing to encourage our communities to step up and look out for our elderly, vulnerable neighbours and friends.
“Please take time to talk to older neighbours and relatives about their security and things they need to do to stay safe. Pop round to check they’re OK, keep an eye out for any suspicious activity near their homes, and call us if anything seems amiss.”
Anyone with information is urged to call 101 or Crimestoppers on 0800 555111 and quote the crime reference number 20/1880053/21.