That field is calculated on the fly.
If your cash is out of balance with the GL and it was fine last month, a likely culprit is that you did something that effects cash dated in March but posted it in April.
This is assuming you made no Transaction Types that begin with O in cash or the checkbox that says that the GL updates cash accounts wasn't unchecked in CA Setup.
Check the GL Account History screen for that account and see if the YTD Balance for the period you are reconciling is the same as the GL balance showing in the Bank Reconciliation screen.
You can run the Bank Reconciliation report and check the Statement to GL Exceptions at the end. Maybe there is a clue there.