May 18, 2015 11:15 AM

First comes love, then comes baby, then comes … a harsh dose of reality.

In an exclusive clip from Monday’s episode of Teen Mom OG, Catelynn Lowell is finally ready to make her relationship with longtime boyfriend Tyler Baltierra official.

When she goes to her wedding gown fitting just two months after having daughter Novalee Reign, though, she learns that the dress she bought before her pregnancy is a lot more snug than she’d hoped – it won’t pull down over her hips, and the bridal shop attendant tells her she’ll have to drop four dress sizes.

When Catelynn returns home, Tyler is visibly surprise by how disciplined Catelynn, who says she’s already lost 20 lbs., will have to be to get ready for their big day.

Although Tyler tries to make helpful, practical suggestions, it’s the bride’s way or the highway as Catelynn vows to hit the gym to get in shape before the wedding.

RELATED: Catelynn Lowell & Tyler Baltierra Share Bittersweet Memories of Carly as They Bond with Baby Nova on Teen Mom OG

Teen Mom OG airs Mondays (10 p.m. ET) on MTV.

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