Is it grammatically correct to say “I was enthused” rather than “I was enthusiastic”?
If so, what is the difference between the two?
The answer is one of preference, not grammar.
Enthused is a back-formation. Because it’s newer, some people may be more used to hearing enthusiastic. Also, as the quote below indicates, the two words can have different implications.
Quoth the grammarist:
… problems arise when enthused is used as a past-participle adjective in place of enthusiastic, which is a perfectly good, and far older, word. Plus, the adjective enthused can carry an ironic tone and is difficult to use in earnest.