The 22-year-old put her side ahead from the spot but successive strikes from Jenna Dear, Emma Johnson, and Ellie Gilliatt ensured the home side came from a goal down to secure a 3-1 lead.
In the 76th minute, Umotong pulled one back for the Yellows but got sent off a minute later which left her side crashing to their four defeat of the season.
The Nigerian who bagged 13 goals in 17 games to grab the top scorer award last season, has now scored four goals in six games this term.
The defeat sees Oxford falls to the ninth on the league table with two points from six matches. They will face fourth-placed Millwall Lionesses at St Paul's Sports Ground on April 22.