Why did widowed Medieval Queens often "retire" to a convent? And did they actually become/live as nuns? What was their life like there?

One major reason that widowed queens did this is that they had religious devotion. I think it's generally difficult for modern westerners, particularly Americans, to conceptualize religious devotion of the Middle Ages. I'm not an expert on that specifically so I shan't embarrass myself trying to go into detail, but I would note that we often see and portray the most zealously Christian people today as hypocrites, people who seek power and riches despite all of the scripture against that sort of thing; as a result, depictions of medieval Christianity tend to have everyone sort of benignly and aesthetically Christian, wealthy characters tend to be kind of atheistic, and the true believers are nasty old priests who want to use religion as a cudgel to get more power. This is ... not historically accurate, as there is loads of evidence of medieval people being devout in harmless and normal ways throughout their lives.

But when it comes specifically to queens, religious devotion was an important part of their "job". This was particularly true in the earlier Middle Ages, when many European kingdoms were still converting to Christianity and the religion was to some extent seen as a peaceful one, not compatible with being a fierce warrior. As a result, the queen needed to be a counterweight to her husband to allow him to do aggressive things while still maintaining, basically, a good level of Christianity as a couple. Some major parts of being publicly pious were attending religious services (of course), working with bishops, visiting holy sites in their realms, paying attention to local saints (both living and non-living) ... and creating and patronizing convents. Queens possessed money and land, and they were able to use them to have convents built and endow them so that they could last in perpetuity. Surplus princesses might be sent to live in them and become their abbesses, extending the royal family's piety, or they might found and endow their own if they were adults. And then, having these close relationships with certain convents, queens could retire to them after they "retired" from being queen consorts, where they could leave in peace and relative luxury.

But there are other factors as well. Convents were safe places: while some were presided over by a bishop, or paired with and subordinate to monasteries, many were independent estates managed by women, and they of course were religious institutions, with the protection that duty gave. A widow who went into a convent could avoid being forcibly remarried to another king or nobleman. They could also be prisons - some queens were forced to enter them once they were widowed to stop them from remarrying, or from exercising power on their own. As women with money and many, many powerful personal connections that they'd built over the course of their lives, they had the potential to be very politically active, which could easily be troubling to their husbands' successors (whether it was their own sons or some other man). Later, there would be more of a tradition that dowager queens should not remarry due to their relationships with kings - that it would be disrespectful for them to be sexually involved with another man - in which case a convent would be a good way to prevent that.