You have added a new explanatory variable to your model. Unfortunately, this new variable is multicollinear with the pre-existing explanatory variables, causing multicollinearity in your model.

1) Adding variables that are collinear to the pre-existing explanatory variables does not always imply higher standard errors.

2)Confidence intervals of coefficients are unambiguously larger.

3) The new regression coefficients are no longer Best Linear Unbiased Estimators(BLUE).

4) Standard errors of coefficients are unambiguously higher.