We don't know the exact circumstances alluded to in this particular poem, but we can offer a general context for Wyatt's poetry. He was a courtier to King Henry VIII of England in the first half of the sixteenth century. This means that he served the king and other powerful men in his court, such as Thomas Cromwell. He was close to power, didn't have to labor for living, and was deeply enmeshed in court politics and the courtly life of romantic intrigue, dances, masques, music, and poetry.
In this poem, the speaker expresses his disbelief at the inconstancy of a beloved, asking over and over " Is it possible?" He is reeling from the change in his loved one from kindness to cruelty. He wonders how she can "bring that low’st that was most aloft."
It is possible that the poem refers to his wife, from whom he separated after accusing her of adultery. It could also refer to Anne Boleyn, with whom he was rumored to have had an affair. But since we don't know, it is safest to assume it is a general commentary on the love intrigues and sudden betrayals frequent in a royal court.