**Neither a teacher nor a native speaker.**
I am going to Germany next month.
This means it's 100% sure that you're going.
You've already gotten your tickets, and everything is fine.
I am going to go to Germany next month.
You plan to go to Germany, and it's pretty sure, but not as sure as in sentence 1.
Sometimes these tenses mean the same.
So it could be possible that someone says these three sentences although he means exactly the same:
I am going to Germany next month. -> 100% sure
I am going to go to Germany next month. -> 90% sure
I will go to Germany next month. -> 80% sure
These % are just examples; there isn't such a specific rule.
As said, that's the "theoretical" rule, but in "practice" all three sentences could mean 100%.
I would say:
I was in (...) classes when you called me.
Unfortunately, I cannot tell you if you should use had.
Furthermore I believe it could be better to add a word where I put the (...).
I was in my classes...
I was in some classes...
(But this is just my personal opinion; I don't say it's wrong not to add something )
Same, I don't see a reason to use had.
I went to (...) computer classes.
Student or Learner