You see, her widow's garments would have identified her to the outside world as a widow. But she took them off from her, ...
Judah thought she was a harlot because of the way she was dressed. That is why it specifically said she took off her normal clothing. Now what was in her heart? She wanted a husband that was promised to her, and the way she decided to get it was to seduce Judah, and so she seduced him using clothing. That must have been, for the times anyway, pretty revealing, because he fell right into the trap.
Now you come to her motives for what she was doing here. She saw that Judah was tricking her, or keeping her from what was rightfully hers. Whether she was thinking that she would be barren (because, remember, she was probably by this time in her early twenties and beginning to feel like she was an old maid) and that Judah had a responsibility to her and he was not fulfilling it. Like Sarah and Rachel, and others, she was going to somehow take matters into her own hands and work this out. So she decides to pose as a harlot.
