If the plant was either kept outdoors at the local shop or grown outdoors and only recently transported to the local shop from an outdoor grower, it could be experiencing some transition shock. I found that a couple of my larger Alocasia went through this even before I had repotted them or established a watering routine. They dropped a few leaves and also "sweat" quite a bit while adjusting to the indoor environment. They seem to notice a lot more than other aroids, IMO.
It's hard to determine if a cup of water is enough for having it five days. Was it dry when you got it? Was it moist when you got it and you used the cup of water once it dried out? I have found that I do not water alocasia through the drainage hole like I would a philodendron. Maybe others will offer different opinions, but I've found that these plants do better indoors with smaller sips of water rather than a thorough soaking. Just enough to moisten the soil but not enough to flow through the hole in the pot. When my big alocasia odoras are outside in the summer, I can soak them with a hose and it's fine. Humidity helps, too. I mist my alocasia or give them a leaf bath. In the wild, these plants wouldn't just be receiving moisture at soil level. It could be in my head, but I think they like to feel moisture on their leaves. I use distilled or rainwater.
Also, these guys are super sensitive to changes in temperature. I'm not sure where you're located temperature-wise (might have missed that!). Last week my husband opened the window with temps in the 50s and one of my alocasia in a pot of 4 turned all sorts of crazy shades and then keeled over. I've never had it go dormant because I've babied it, so I'm sure it's awfully confused now.
If you haven't done anything wild with potting or watering, I'd keep a close eye on it for a week or so and see if it evens out. Maybe turn some awareness to the soil and reach your finger down in the pot to determine when to water. There are co flirting opinions on moisture meters, but I use mine regularly, especially for larger pots that need to be tested deeper than my finger can reach, or for pots densely packed with stems. I look for an alocasia to be around a 2 before I water it. Philos and monstera in my home are fine with anywhere from 1-4 but IME the alocasia are more specific about what they want. You'll find what works for your conditions, too.