Sie wird bei dem Gedanken das Herz schwer.

[GooDDmann]

could anyone help to translate that please!

[SteffiBookworm]

Ihr wird bei dem Gedanken das Herz schwer.
With the thought her heart becomes sad. (?)

[GooDDmann]

great thanks to correct it

[SteffiBookworm]

You're welcome. :)

[slamRN]

"It makes your heart heavy at the thought."

[Kit845299]

"Ihr wird bei dem Gedanken das Herz schwer." (also: "Ihr wird bei dem Gedanken schwer ums Herz.") = "Her heart sinks at the thought."/ "Her heart saddens at the thought."

[gatiquo]

she will make her heart heavy with these thoughts

[tu.8zPhLD72zzoZN]

Expressions don't always translate word for word. How about "It makes your heart heavy at the thought." but in German it is plural: thoughts.
http://dictionary.reverso.net/german-english/gedanken

[stepintime]

Not "thoughts" plural, but "thought" dative singular.
And "her heart", as SteffiBookworm already pointed out; and yes, it's "ihr" ("to her") instead of "sie".