Marc Anthony Throws Tantrum Over Jennifer Lopez's New Beau!

Jealous much?

Marc Anthony is up in arms that his ex Jennifer Lopez has moved on since their July split with a much younger man. A source tells the new Us Weekly, on stands Friday, that 43-year-old Anthony is "going nuts" over Lopez's relationship with dancer beau Casper Smart, 24.

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"He has fits and tantrums and is very stressed... [He's] lost a lot of weight," a source adds. (A rep for the Latin crooner counters this, telling Us Anthony has "been the same weight most of his life.)

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But pals of Lopez, 42, aren't crazy about Smart either. "They don't feel Casper is good enough for her," a source tells Us. "They are worried because she really puts her heart out there for love."

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For more on Anthony's jealousy and Lopez's fast track romance with Smart, check out the new Us Weekly, on stands Friday.