It's very likely that the pain in your side was unrelated to your bends since the joints are more usually affected. Usually pain in the side, without any other indicators, is a symptom of back strain from the heavy tank or underwater posture (preceded by lugging heavy luggage and sitting in a cramped airplane seat). Without any other symptoms, the new pain in your side may be just back strain. The test is whether it resolves when you breathe oxygen or better yet get recompressed (in a chamber, since underwater recompression isn't a good idea if there's a chamber handy). The chamber test may be costly, but hopefully covered by dive insurance. The oxygen test might be free if you can find a willing dive op to give you a few hits from their tank, but save that for the end of the day so they can refill it before going out again.
That all said, I do believe there are online-only nitrox classes. Any dives associated with a class would only determine that you can dive within your MOD (maximum operating depth). Hands-on analysis of a tank is helpful, but you can get that "class" for free on any dive boat, just watch the DM analyze the tank and ask questions. Besides, there are various types of analyzers, so learning how to use one doesn't guarantee you'll know how to use a different one anyway.
As Christi said, for multiple days of diving, and especially in deeper areas like Cozumel (though not past your MOD of course), nitrox is a good idea anyhow for a safety factor (this concept has some detractors but they're just wrong) and/or extending bottom time, and at least anecdotally, making you feel less tired after a day of diving.
