I am using STATA 16.1 and trying to estimate the following household (hhid) model.

reg D.y peak D.x peak#D.x

(I actually run reg diff_y peak diff_x peak_diff_x, where, diff_y is D.y and alike, and peak_diff_x is peak*diff_x , peak is dummy, as D. operator is not allowed with factor variable I have to run this way.)

I want to calculate the effect of D.x in peak (agricultural season) by adding coefficient of D.x and peak#D.x

However, it seems to me STATA is giving me wrong first difference for my purpose for seasonal data.
Please let me know where I go wrong?

I got the following first difference as

xtset hhid wave

gen diff_y = D.y
gen diff_x= D.x


input int hhid float wave byte peak float(y diff_y x diff_x)
2 1 0 2.6390574 . 1.568616 .
2 1 1 2.833213 . 1.568616 .
2 2 0 4.0943446 1.2611313 1.609438 .04082203
2 2 1 4.477337 1.6441236 1.609438 .04082203
2 3 0 5.252274 .7749367 1.609438 0
2 3 1 6.042633 1.565296 1.609438 0
3 1 0 4.4426513 . 1.0296195 .
3 1 1 5.105946 . 1.0296195 .
3 2 0 3.583519 -1.5224266 1.0986123 .06899285
3 2 1 4.158883 -.9470625 1.0986123 .06899285
3 3 0 4.553877 .3949938 1.0986123 0
3 3 1 5.631212 1.4723287 1.0986123 0