Woke up with bites on my butt. Bedbugs?

Could be. Or it could be you were bit the day before and there was just a delay before the red marks showed up. Were you outside sitting in the grass the day before? Check your mattress for signs of bed bugs.
Maybe not. Bed bug bites may have no sx's/signs except for 2 small skin puncture points. A reaction may show from an hr to several days later. It can cause itchy, pink, swollen, lesions w clear center. If hypersensitive blisters may occur. Telltale sign is linear track of lesions from sequential feedings – but doesn't always occur. Bites are in areas exposed while sleeping-arms, shoulders, neck ; legs.
Not sure, depends. It depends on what the bites look like. Bedbug bites have a blotchy, smudgy, red appearance. They also remain close to their hosts, and in beds and couches, and they tend to have the smell of over-ripe rasberries. Other insects and parasites can cause bites. Scabies have a dark, wavy line burrow with a bleb at the end, and lice and ticks also bite. Itching is also common.