syllable stress, Present versus Pretérito Perfeito (Simple Past)

In the practiceportuguese.com audio examples,
it sounds to me like the stress of the
syllable changes between these tenses:

Present vs. Simple past (Pretérito Perfeito)

when using the eu conjugation. For example with:

eu acho

the stress is on the first syllable but
with:

eu achei

the last syllable is stressed. For this last
example, just look up the audio example for:

Eu achei que sim

Is this correct? Does the syllable that
is stressed change between the present
and simple past for “eu” conjugations?
It sounds like it to me.

Or am I understanding this wrong?

Thanks.