If you only flipped heads once, it would be natural to me to say just that. "I only got heads once." Or you could say, "I only got one heads". If you say it this way, it specifies that you only got heads one time. If you say, "I got a heads" it might sound more present tense, like you just barely got heads finally after so many tails! Like, "finally! I got (a) heads! "

We normally say "I got heads," "all my flips were heads," or "I got x heads, y tails." Hope that helps.

That does sound strange. I understand it, but it's more common to say something like, "I've gotten heads every single time!" or "Every time I flip this coin I only get heads!" That would be a cool coin, though! I hope that helped!