In one sense, the only difference is that It was is past tense, and It's (It is) is present tense. I can also make one change in the second sentence that may help the comparison: It's my fault that I asked a stupid question. Then the sentences become even more similar, and maybe it's easier to see that there is just a difference between past and present tense.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.