I'm estimating a diff-in-diff model with:

Code:
xtdidregress (y) (treated) [pweight=my_weights], group(groupnum) time(year)
Then I'm replicating the results with:

Code:
xtset groupnum year
[..]
xtreg y ib0.treated i.year [pweight=my_weights], fe vce(cluster groupnum)
I obtain the same coefficients and the same standard errors.

However, the latter model gives me an R^2, which is very low:
Code:
R-squared:                                     
     Within  = 0.0493                       
     Between = 0.0106
     Overall = 0.0026
Do I have to worry?

Do I have to look for R^2 in a diff-in-diff model at all?