The speaker has a beer and then lets loose, venting about everything. The drink seems to free them from the usual rules. Nobody expects them to lie to people, so for once they can say what they think. But the poem doesn't treat this as a clean victory. "I put on feeling" is a telling phrase: it suggests the emotion might be worn like a costume, something chosen or exaggerated rather than simply felt.
The last two lines turn sharply. Being "called up for this strange use" makes the speaker sound almost drafted into a role, and that role is "to shit in the well," a crude image of fouling a shared source of water. So the outburst, however liberating, may have poisoned something the speaker and others depend on. The mood is raw and uneasy, a mix of release and regret. The poem seems to ask whether blunt honesty is always a good thing, or whether it can simply be a way of doing damage.