STOP BREEDING HER UNTIL YOU WEIGH HER!! I understand that you want eggs and fast, I was the same way. If she only laid one egg, and it was a BAD egg at that, then SHE IS NOT READY TO BREED. Something is WRONG. Healthy adult cresteds lay 2 eggs, and RARELY are they infertile. The egg is most likely under calcified, meaning she is either not eating right, she is too young, too light, or all of the above. If you let her mate before you know her size/age/weight/eating habits are correct, and she lays bad eggs, then you are just stressing her out by mating her because it is for no reason. Under calcified eggs are yellow and grow mold easily, if that is what the egg looked like, yellow instead of bright white, then it was under calcified, again I stronlgly suggest holding off breeding until you get her weight. Just my $0.02, this is not an attack, just think of your animal before your excitement of eggs. And what will you do with the babies??
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2.3.4 crested geckos (Speckles, Sandy, Squirt, Sunshine, Soldier and four un-named babies)
0.0.1 Striped Cal. Kingsnake (Snuggles)
1.1.0 Green Iguanas (Sisco,Speedy)
0.1.0 Rose Hair Tanrantula (Sweets)
RIP Star and Sal