It could be a reaction to some kind of medication -- did you take any kind of medicine before you went to bed? I'm allergic to Panadol and I would get similar rashes if I took them and it would be made worst by scratching.
The marks look like scratches by fingers. Did you scratch your back during sleep? It doesn't look like bedbug or allegies reaction to me. Perhaps you can monitor for another few more nights to see if it is still itchy.
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Did you sit against something last night, like a chair or anything that might have left marks? Agree with diana, looks more like scratch marks.