Older dog vomited foam three times, now quiet and holding a prayer position

What they asked

Hi, I’m looking for advice about my dog Charlie. He is an older Chihuahua/Beagle mix. Earlier today, around 2–4 PM, he vomited three times within a few minutes. It was mostly foamy/slimy rather than a normal food-filled vomit. He has not vomited again since then. Since vomiting, he has been acting noticeably different from his normal behavior. He is more lethargic and spends more time lying down. Normally around 7–9 PM he gets very excited and aggressively begs for his food, barking and getting up constantly, but tonight he mostly stayed lying down and just watched us. He was also initially picky about food, which is unusual for him because he normally eats almost anything. He eventually ate some beef and eggs after we mixed them together and has kept that down. He is still responsive and does not seem severely ill. If we call him, he follows us, gets up to go potty, and his ears perk up when he wants treats. He has also eaten some grass. His gums look normal and pink. The thing that concerns us most is that he has repeatedly been doing a downward-dog/prayer-like position. Normally when he stretches like this, he immediately gets up or walks away, but today he has held the position with his butt in the air and stayed that way until someone touched him, then he lay down. He has done this several times. His stomach also seems possibly a little more bloated than normal, although we're not completely sure because we don't know what his normal abdominal size should look like. It does not seem extremely swollen, and he doesn't appear to be in severe pain. We haven't seen him get into anything obviously dangerous. There is nothing in the rooms where he has been that we know of that could have seriously harmed him. Overall, he is responsive, walking when he chooses to, going potty, eating some food, and hasn't vomited for several hours, but he is definitely more lethargic and is behaving differently than normal. We are concerned about whether this could just be an upset stomach or whether the vomiting, possible bloating, and repeated downward-dog position could indicate abdominal pain or something more serious.